This invention relates to solid propellant compositions and to an improved plasticizer system for use therewith. In a more particular aspect, this invention concerns itself with a family of novel straight-chained, saturated polyester-ethers of particular molecular structure and to their use as internal plasticizers for isocyanate-cured propellant compositions.
The increased interest and utilization of solid propellant compositions has spawned a considerable research effort in an attempt to improve their ballistic and physical properties. Generally, solid propellants are composed of one or more organic or inorganic oxidizers dispersed in a resinous binder matrix which may also function as a fuel. Typical oxidizers are ammonium perchlorate or HMX (cyclotetramethylene tetranitramine), both of which are well known in the art. Various resinolus components, such as hydracarbons, polyesters, polyurethanes and other like materials may serve as the binder/fuel matrix. A supplemental fuel component, such as finely powered aluminum, may be used also. Other additive components, such as anti-oxidants, burning rate modifiers, wetting agents, anti-foaming agents and plasticizers may be added to the propellant composition, if desired. Dibutylphthalate, dioctyl adipate or triacetin are often employed as inert plasticizers in combination with the resinous binder material.
Plasticizers are used in rocket propellants for a number of purposes. These include processing assistance by incorporation of fluid materials in the propellant mix, the improvement of low temperature flexibility and the improvement of mechanical properties and ballistic characteristics. In using solid propellants, however, a problem exists in the use of plasticizers because of the tendancy of plasticizer molecules to migrate or evaporate out of the propellant composition during storage. Either of these processes results in chemical changes in the composition which are harmful to the propellant and to other inert parts of the rocket motor. On long term storage, changes in the mechanical properties of propellant often occur, particularly near the liner-propellant bond. Other negative effects which are associated with the use of plasticizers include crystallization at low temperatures, a tendency to soften the propellant excessively at high temperatures, and the fact that the plasticizer may migrate into the liner from the uncured propellant much more rapidly than it does from a cured propellant system. As a consequence, a continuing research effort has been conducted in an attempt to solve the migration problem associated with propellant plasticizers and provide a plasticizer that does not migrate rapidly, even in an uncured propellant. One research effort involved the use of a material known as ZL-496. This material, ZL-496, is a polybutadiene with a molecular weight of approximately 3,000. The use of ZL-496 as a plasticizer was contemplated because its molecular size is quite large and it was believed that its polymer chain entanglement would prevent migration. Unfortunately, however, it was found that even plasticizer molecules as large as ZL-496 have a tendency to migrate out of the propellant composition with a resulting shortening of the shelf life of the rocket motor.
In furthering the research effort referred to above, however, it was unexpectdly discovered that a family of straight-chained, saturated polyester-ethers of particular molecular structure could be utilized as plasticizers in the fabrication of solid propellants. This novel plasticizers of this invention replace the conventional plasticizers generally utilized in conventional composite propellants, especially those having a binder base which uses an isocyanate cure reaction for its curing system. The resulting propellant exhibits a minimum amount of plasticizer migration during the normal shelf-life storage period of the propellant.